1. Field of the Invention
The present invention relates to a communication system for performing a synchronous broadcast communication such as a distributed processing system for executing jobs by using, for example, a plurality of computers.
2. Description of the Related Art
In recent years, computer system have become larger and more complex, involving diversification of requests and technical sophistication. For this reason, demands are increasing for a distributed processing system for executing jobs by using a plurality of computers.
Each computer used in the system may be called a "communication main body" because it exchanges (or transmits and receives) information with the other computers.
In this type of a distributed processing system, when the computers, i.e., communication main bodies, are connected to one another by low reliable communication lines, message missing or loss is likely to happen. To prevent the message missing, it is desired to specify the missed messages and transmit them again.
Therefore, in the conventional method, in order to prevent the message missing, the transmission line is multiplexed and the received messages are compared to one another, thereby to improve reliability. Alternatively, a virtual 1-to-1 communication line is set between two application programs, and the message for reception confirmation is exchanged between these programs.
However, when the multiplexed communication line is used, the processing speed of the system decreases because the control of the multiplexed communication line is complex and there are message-reception waiting periods.
Further, when the virtual communication line is used, the processing speed of system decreases and, moreover, the broadcast communication can not be accomplished because the message for reception confirmation must be awaited during the normal communication time.
As a method of retransmitting the message when message missing occurs, there has been proposed a method of adding the message for reception confirmation into a transmission message, and a method of ordering all messages by a host computer, or adding a serial number to each message for identification. In the first method, when the transmitter which incurs message missing transmits the next message, a reception error is detected and the missed message is retransmitted. There is the possibility that the missed message is not received unless the next message is transmitted and therefore the processing interruption time lengthens. The second method entails the problems that the host computer makes a bottleneck to processing speed and reliability, the performance of the system is governed by the performance of computer, the processing for dissolving a trouble in the host computer must be carried out, and messages not relating to actual processing are required.
As described above, the conventional system employs a multiplexing communication line or a virtual communication line in order to prevent a broadcasting message from being missed. The control of communication lines is inevitably complicated, the overhead of processing the message for reception confirmation increases and the time for waiting for the confirmation is long. Consequently the processing speed of the system is low. Further, a method of retransmitting the missed message has disadvantages such as lengthening the processing interruption time, and making a bottleneck in the processing speed and reliability.
Accordingly, there is required, in order to perform actually the broadcast communication, a communication apparatus wherein the virtual communication line is unnecessary, processing overhead such as waiting is small, the message can be quickly retransmitted even if message missing occurs, and no bottleneck exists with respect to reliability and processing speed.